With the rapid expansion of the use of wired/wireless communication including Internet communication, issues related to the security of communication networks have become increasingly important in terms of the protection of the important secrets of nations, corporations, and banks and the protection of personal privacy. The asymmetric public key cryptosystem which was developed in the 1970s and is currently being widely used in communication systems, such as the Internet, etc., is a method designed to encrypt information by using a mathematically complex problem as a public key and to decrypt the information by using the solution to the problem as a private key, and is based on a mathematical “computational complexity” in principle.
As a representative, the RSA public key cryptosystem developed by the three persons, i.e., Rivest, Shamir, and Adleman, uses the difficulty of factoring very large numbers into primes. In other words, from a mathematical aspect, the factoring problem is based on the fact that as the size of a problem increases, computational time increases exponentially, with the result that when a sender and a receiver use the problem of factoring a sufficiently large number as a public key, it is practically impossible for an eavesdropper to decrypt a cryptogram. However, the security of the cryptosystem based on computational complexity is being questioned due to the development of more elaborate algorithms. Furthermore, as Peter Shor at AT&T developed a factoring algorithm using a quantum computer in 1994, it was proven that the RSA cryptosystem could be fundamentally cracked if a quantum computer was developed.
Quantum cryptography technology proposed as an alternative intended to overcome the above security problem has security based on the principle of quantum mechanics, i.e., the fundamental law of nature, rather than mathematical computational complexity, and thus it makes eavesdropping and monitoring difficult, with the result that it has attracted great attention recently. In other words, quantum cryptography technology is a technology designed to absolutely securely distribute a secret key (a one-time pad) between a sender and a receiver in real time based on the law of quantum physics, such as “the nonclonability of quantum information,” and is also known as “quantum key distribution (QKD) technology.”
The first quantum cryptography protocol was presented by C. H. Bennett at IBM and G. Brassard at the University of Montreal in 1984. This protocol named after the creators as the BB84 protocol uses four quantum states (for example, the polarization states of a single photon) constituting two bases.
One example of quantum cryptography technology is disclosed in the article “Quantum Cryptography Technology,” Electronics and Telecommunication Trend Analysis, Vol. 20, No. 15, October 2005.
The prior art relates to quantum cryptography technology using the quantum system of a two-dimensional Hilbert space, i.e., a qubit (a quantum bit).
Conventional research and development into quantum cryptography technology focus chiefly on the efforts to improve the sensitivity and reliability of the reception of a quantum cryptogram. Accordingly, quantum cryptography technology has high barriers to the access of general users, which has been the reason for which quantum cryptography technology cannot be extended to various industrial fields notwithstanding that quantum cryptography technology has desirable security performance.